Long-lived quantum memories with rare-earth ions doped in crystals

Imagine a world where information is carried not by electric currents in wires, but by individual particles of light. These particles, known as photons, are the ideal messengers for a new kind of internet that promises unbreakable security and unprecedented computing power. However, light has a fatal flaw: it cannot be stopped. Once a photon is sent down a fiber optic cable, it travels until it is lost or absorbed. It cannot wait. This makes building a global network impossible, because signals weaken over distance, and unlike a traditional signal that can be copied and boosted, a quantum signal cannot be copied without destroying its delicate information. To solve this, scientists need a way to catch a flying photon, hold it still, and release it later exactly as it was. This device is called a quantum memory. It acts as a pause button for light, allowing the network to synchronize its operations and extend its reach across continents.

For years, researchers have searched for the perfect material to serve as this pause button. They needed something solid, easy to build with, and capable of holding a quantum state for a long time without it fading away. One promising candidate has emerged from the world of crystals: rare-earth ions. These are atoms of elements like europium or praseodymium that have been embedded into a crystal lattice. The unique thing about these atoms is that their outer layers act like a shield, protecting their inner electrons from the noisy environment of the crystal. This protection allows them to hold onto their quantum state for surprisingly long periods. Yet, even with this natural shield, the environment still interferes. Tiny fluctuations in magnetic fields, caused by the random motion of atoms in the crystal or the Earth's own magnetic field, cause the stored information to scramble and disappear.

A team of researchers at the University of Science and Technology of China has now taken a major step forward in taming this interference. In a comprehensive review of recent experiments, they describe how researchers have managed to extend the life of these quantum memories from seconds to minutes, and in some cases, to hours. The key to this success lies in two clever strategies. First, they identified a specific magnetic field strength where the atoms become naturally immune to magnetic noise. It is as if the atoms find a "sweet spot" where the magnetic push and pull cancel each other out perfectly. Second, when the atoms still begin to drift, researchers apply a rapid series of radio-frequency pulses. These pulses act like a gentle nudge, constantly realigning the atoms before they can lose their way, effectively freezing the information in place.

The results of this work are measured in three distinct ways, each representing a different level of difficulty. The first measure is the "spin coherence time," which is simply how long the atoms can hold their quantum alignment before they start to drift apart. Using crystals doped with europium ions and applying the magnetic sweet spot along with the realigning pulses, researchers achieved a record-breaking coherence time of nearly six hours. This means the atoms remained synchronized for a duration far longer than any previous experiment with this type of material.

The second measure is "light storage time." This is the duration for which an actual beam of light can be captured, stored as a vibration within the atoms, and then released back out as light. In earlier experiments with praseodymium ions, scientists managed to store light for about 42 seconds. By switching to the more stable europium crystals and using a different storage method that relies on a specific pattern of atomic energy levels, researchers extended this time to over 52 minutes. They demonstrated that a pulse of light could be held for nearly an hour and then retrieved with its wave pattern intact, proving that the information had not been scrambled.

The most difficult challenge, and the one that brings us closest to a working quantum internet, is "quantum storage time." This involves storing not just a bright beam of light, but a single particle of light, or a single photon. Storing a single photon is incredibly hard because the process must be so efficient and quiet that no extra noise is added, or the single photon will be lost in the static. Previous attempts struggled to keep a single photon for more than a few milliseconds. However, by combining the magnetic sweet spot, the realigning pulses, and a new method that avoids creating extra noise during the storage process, researchers led by Lv et al. successfully stored single-photon-level inputs for over 27 seconds. At this duration, the stored photon retained its quantum nature, proving it was not just a classical signal but a true quantum memory.

These achievements are not just theoretical milestones; they point toward a practical future. The researchers note that these crystals are compatible with existing technology, meaning they could be integrated into small devices. This opens the door for "transportable quantum memories," where a device containing a quantum memory could be loaded onto a truck, a drone, or a satellite to carry quantum information between distant cities or even across space. The ability to hold a quantum state for an hour means that a vehicle could travel hundreds of kilometers while keeping the information safe, bridging gaps that fiber optic cables cannot cross. While challenges remain, such as improving the efficiency of capturing the light and making the equipment smaller, this work establishes that long-lived, solid-state quantum memories are no longer a distant dream but a tangible reality.